This invention relates to systems for recording and reproducing four separate channels of information on a medium having only two independent tracks, and more particularly to apparatus for reproducing such information and presenting it on four loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. More specifically, the present invention is concerned with an improved logic for multichannel matrix decoding systems of the type described in the above-mentioned co-pending applications for improving the realism of the reproduced sound.
Briefly, in a matrixed quadraphonic system several channels of sound, say four, are combined by an encoding matrix into two channels whereby they may be recorded on a two-channel medium, such as a disc record, or transmitted over two channels, such as FM-multiplex radio. A suitable encoder for this purpose is illustrated in FIG. 8 of the aforementioned U.S. Pat. No. 3,821,471, and in application Ser. No. 384,334. To recover as best as possible the original four channels, the two composite signals are applied to a decoding matrix which is operative to produce four new signals each predominantly containing one of the original four signals, but also containing fractional portions of others of the original signals transferred from the original channels to the adjacent or opposite channels; the fractional signals are referred to hereinafter as side-effect signals. For some directions of sound arrival to be portrayed by the four-channel array the side-effect signals are harmless; in other cases, however, they tend to diminish the realism of the original four-channel sound. In order to counteract the effect of the side-effect signals, variable-gain amplifiers are used in the four output channels, and a logic network is provided to sense the presence of the desired signals and/or the presence of transferred side-effect signals and to develop control signals for adjusting the gains of the variable-gain amplifiers rapidly and continuously in response to program sounds in a manner to diminish or eliminate the perception of side-effect signals. The aforementioned U.S. Pat. No. 3,821,471 discloses a logic network whose action is based on wave-matching and amplitude comparison techniques. Although this previous decoder produces good channel separation and substantially eliminates side-effect signals, the circuitry is relatively complex, and consequently more expensive, and is not as readily adaptable to implementation by integrated solid-state techniques as would be desired.
It is a primary object of the present invention to obtain equal or greater quadraphonic realism than that attainable by the logic circuitry described in the aforementioned co-pending applications while, at the same time, simplifying and reducing the cost of the circuitry for accomplishing it. A more specific object is to provide a logic network which is readily adaptable to integrated circuit implementation.